The present invention relates to thermographic nondestructive testing techniques for determining the thickness of an object. More particularly, the present invention relates to a infrared transient thermography method that utilizes a synthetic thermal reference in determining the wall thickness of metal turbine rotor blades or the like.
Over the years, various nondestructive ultrasonic measurement techniques have been utilized to determine cross-sectional thickness of cast metal and other solid objects. Conventionally, the object is probed with ultrasonic waves which penetrate the surface and are reflected internally at the opposite side or surface of the object. Based upon the time required to receive a reflected wave, the distance to the opposite (back) side can be determinedxe2x80x94giving the thickness of the object at that point. Unfortunately, conducting ultrasonic measurements of this sort to examine the cross-sectional thickness for most of an object would usually necessitate a cumbersome and time-consuming mechanical scanning of the entire surface with a transducer. In addition, to facilitate intimate sonic contact between the transducer and the object surface, a stream of liquid couplant must be applied to the surface or, alternatively, total immersion of the object in the couplant must be accommodated. Such accommodations, however, are most often not very practical or even feasible for numerous structural and material reasons. For example, ultrasonic systems capable of scanning and analyzing geometrically complex parts are typically very expensive and complicated. In addition, a mechanical scanning of the transducer over the surface of a large object can literally take hours.
Moreover, when conducting ultrasonic measurements on certain metal objects, the internal crystal orientation and structure of the metal can cause undesirable noise and directional effects that contribute to inaccuracies in the acquired data. This inherent limitation of ultrasonic measurements proves to be a serious drawback when testing components constructed of crystalline or xe2x80x9cdirectionalxe2x80x9d metals such as often used in contemporary turbine airfoils.
In contrast, infrared (IR) transient thermography is a somewhat more versatile nondestructive testing technique that relies upon temporal measurements of heat transference through an object to provide information concerning the structure and integrity of the object. Since heat flow through an object is substantially unaffected by the micro-structure and the single-crystal orientations of the material of the object, an infrared transient thermography analysis is essentially free of the limitations this creates for ultrasonic measurements. In contrast to most ultrasonic techniques, a transient thermographic analysis approach is not significantly hampered by the size, contour or shape of the object being tested and, moreover, can be accomplished ten to one-hundred times faster than most conventional ultrasonic methods if testing objects of large surface area.
One known contemporary application of transient thermography, which provides the ability to determine the size and xe2x80x9crelativexe2x80x9d location (depth) of flaws within solid non-metal composites, is revealed in U.S. Pat. No. 5,711,603 to Ringermacher et al., entitled xe2x80x9cNondestructive Testing: Transient Depth Thermographyxe2x80x9d; and is incorporated herein by reference. Basically, this technique involves heating the surface of an object of interest and recording the temperature changes over time of very small regions or xe2x80x9cresolution elementsxe2x80x9d on the surface of the object. These surface temperature changes are related to characteristic dynamics of heat flow through the object, which is affected by the presence of flaws. Accordingly, the size and a value indicative of a xe2x80x9crelativexe2x80x9d depth of a flaw (i.e., relative to other flaws within the object) can be determined based upon a careful analysis of the temperature changes occurring at each resolution element over the surface of the object. Although not explicitly disclosed in the above referenced Ringermacher patent, the xe2x80x9cactualxe2x80x9d depth of a flaw (i.e., the depth of a flaw from the surface of the object) can not be determined unless a xe2x80x9cstandards blockxe2x80x9d, having voids at known depths, or an xe2x80x9cinfinitexe2x80x9d (thermally thick) reference region on the object is included as part of the thermographic data acquisition and analysis for comparison against the relative depth values.
To obtain accurate thermal measurements using transient thermography, the surface of an object must be heated to a particular temperature in a sufficiently short period of time so as to preclude any significant heating of the remainder of the object. Depending on the thickness and material characteristics of the object under test, a quartz lamp or a high intensity flash-lamp is conventionally used to generate a heat pulse of the proper magnitude and duration. However, the specific mechanism used to heat the object surface could be any means capable of quickly heating the surface to a temperature sufficient to permit thermographic monitoringxe2x80x94such as, for example, pulsed laser light. Once the surface of the object is heated, a graphic record of thermal changes over the surface is acquired and analyzed.
Conventionally, an infrared (IR) video camera has been used to record and store successive thermal images (frames) of an object surface after heating it. Each video image is composed of a fixed number of pixels. In this context, a pixel is a small picture element in an image array or frame which corresponds to a rectangular area, called a xe2x80x9cresolution elementxe2x80x9d, on the surface of the object being imaged. Since, the temperature at each resolution element is directly related to the intensity of the corresponding pixel, temperature changes at each resolution element on the object surface can be analyzed in terms of changes in pixel contrast. The stored IR video images are used to determine the contrast of each pixel in an image frame by subtracting the mean pixel intensity for a particular image frame, representing a known point in time, from the individual pixel intensity at that same point in time.
The contrast data for each pixel is then analyzed in the time domain (i.e., over many image frames) to identify the time of occurrence of an xe2x80x9cinflection pointxe2x80x9d of the contrast curve data, which is mathematically related to a relative depth of a flaw within the object. Basically, as applied to an exemplary xe2x80x9cplate-likexe2x80x9d object of consistent material and thickness L, a heat flux pulse impinging on an object takes a certain xe2x80x9ccharacteristic timexe2x80x9d, Tc, to penetrate through the object to the opposite side (back wall) and return to the front surface being imaged. This characteristic time, Tc, is related to the thickness of the object, given the thermal diffusivity of the material, by the following equation:
Tc=4L2/xcfx802xcex1xe2x80x83xe2x80x83Equ.(1)
where L is the thickness (cm) of the object and xcex1 is the thermal diffusivity (cm2/sec) of the material.
From empirical observations it is known that after a heat pulse impinges on a plate-like object, the surface temperature observed from the same side of the object (i.e., the front) rises in a fashion that is also dependent on the thickness and the thermal diffusivity of the material. Moreover, from a graph of the time vs. temperature (T-t) history of the surface, one can determine the characteristic time, Tc, in terms of a unique point on the T-t curve, called the xe2x80x9cinflection point.xe2x80x9d This inflection point, tinfl, is indicated by the point of maximum slope on the T-t curve (i.e., peak-slope time) and is related to the characteristic time, Tc, by the following equation:
tinfl=0.9055 Tcxe2x80x83xe2x80x83Equ.(2)
This relationship between the inflection point and the characteristic time, as expressed by Equ. (2) above, is precise to approximately 1% for one-dimensional (1-D), as well as two-dimensional (2-D), heat flow analysis. Once an inflection point, tinfl, is determined from the T-t response, a relative thickness, L, of the object can be determined from Equ. (1) using the known thermal diffusivity, xcex1, of the material and the actual value of Tc from Equ. (2).
In this regard, a more detailed discussion of the heat-flow invariant relationship between the peak-slope time (inflection point) and the material xe2x80x9ccharacteristic timexe2x80x9d as defined above may be found in the Review Of Progress In Quantitative Nondestructive Evaluation, in an article by Ringermacher et al., entitled xe2x80x9cTowards A Flat-Bottom Hole Standard For Thermal Imagingxe2x80x9d, published May 1998 by Plenum Press, New York, which is incorporated herein by reference.
Unfortunately, the above mentioned apparatus and method of U.S. Pat. No. 5,711,603 to Ringermacher et al. only produces xe2x80x9crelativexe2x80x9d depth measurements. It can not be used to obtain a value for the actual thickness of a metal object at a desired point. Consequently, a better method of conducting and processing IR transient thermography that would permit a determination of the actual thickness of metal objects was needed. One such method and apparatus is disclosed in a commonly assigned co-pending U.S. patent application (Ser. No. 09/292,886) of Ringermacher et al. filed Apr. 4, 1999. Basically, the arrangement disclosed therein utilizes a focal-plane array camera for IR image data acquisition and high-power flash lamps to rapidly heat the surface of a desired examined object. A slab object of similar material having portions of known thickness is used as a xe2x80x9cinfinite half-spacexe2x80x9d reference standard within the same image frame (a xe2x80x9cthermally thickxe2x80x9d section of the examined object may optionally be used as the reference). The flash-lamp are fitted with spectrally tuned optical filters that minimize long-wave IR xe2x80x9cafterglowxe2x80x9d emissions and reduce background radiation effects which affect the accuracy of thermal measurements. A predetermined number of IR image frames are acquired and recorded over a predetermined period of time after firing the flash-lamps to develop a temperature-time (T-t) history of the object surface (and the reference standard). Contrast versus time data is then developed for each pixel in the image to determine object thickness at a location corresponding to the pixel position.
In the above method contrast-time data is developed by subtracting temperature-time data of the slab reference standard (or temperature-time data of a xe2x80x9cdeepxe2x80x9d thermally thick reference region on the object) from the temperature-time data of each pixel. Unfortunately, this method suffers from the disadvantage that it may introduce some degree of error when imaging objects that have varying surface uniformity. Moreover, it requires the presence of a slab standard in the image or the use of temperature-time data from a deep reference region on the objectxe2x80x94assuming such a reference region is available. In addition, a special coating must usually be applied to the surface of the object (and the slab standard) prior to imaging to enhance optical absorption and improve surface uniformity.
The present invention relates to a nondestructive testing method and apparatus for determining and displaying the actual thickness of an object through the use of high speed infrared (IR) transient thermography. An improved high-speed IR transient thermography analysis approach is utilized to accurately measure the thickness of an object and provide a visual coded display indicative of its cross-sectional thickness over a desired area of the object. A salient feature of the present invention is that a xe2x80x9csyntheticxe2x80x9d or computed reference, based on actual surface temperature, is used to compute the contrast versus time data needed to determine thickness. Consequently, at least one beneficial aspect of the present invention is that is does not require the use of a separate reference standard or a reference region on the examined object. In addition, when using the transient thermographic technique of the present invention there is no need to apply special coatings to the object(s) being examined. Moreover, the method of the present invention can readily accommodate objects having non-uniform surfaces or varying surface emissivity.
Basically, the present invention makes use of an inflection point in a temperature-time (T-t) response analysis of the surface of a rapidly heated object, preferably obtained from xe2x80x9cfront-sidexe2x80x9d IR camera observations. This inflection point, tinfl, occurs relatively early in the T-t response and is essentially independent of lateral heat loss mechanisms. (Such considerations may be of particular relevance, for example, when working with metals since, due to the high thermal conductivity of metals, the thermal response of a metal object is fairly quick and, consequently, the time available for obtaining thermal data measurements is usually short). The inflection point, tinfl, is extracted from thermal data acquired over a predetermined time period from successive IR camera image frames. Preferably, this time period is at least somewhat longer than an anticipated characteristic time, as obtained from Equ.(1), based on an estimation of the thickness of the object being evaluated.
In accordance with the present invention, the inflection point, tinfl, is determined by utilizing pixel contrast data that is based on a xe2x80x9csyntheticxe2x80x9d or computed thermal reference instead of a xe2x80x9crealxe2x80x9d reference such as a slab standard or a suitable region on the examined object. This computed synthetic reference represents the surface temperature of an object as a function of time for one-dimensional heat flow into a semi-infinite medium (half-space) and is given by the following relationship:
Ts(t)=A[txc2xdxe2x88x92(txe2x88x92xcfx84)xc2xd]xe2x80x83xe2x80x83Equ.(3)
where Ts(t) is the surface temperature of the synthetic thermal reference as a function of time t, A is a parameter selected to match actual surface temperature on an object surface at a location corresponding to a selected analysis pixel in an acquired IR image, and xcfx84 is a duration of heating the object before acquiring image frames.
Essentially, the reference temperature-time data provided by Equ. (3) describes a xe2x80x9csyntheticxe2x80x9d half-space thermal decay based on an initial temperature, A, at a particular location on the surface of the object. As described in greater detail below, the xe2x80x9csyntheticxe2x80x9d thermal reference data as obtained from Equ. (3) is first computed for each (x,y) pixel location of the imaged object and then used to determine contrast as a function of time for each pixel. Moreover, as a further advantage, determination of the synthetic thermal reference data is not dependent upon the nature or characteristic of the particular material or metal being evaluatedxe2x80x94as it is not a parameter in Equ. (3).
As illustrated by FIG. 1, the apparatus accordance of the present invention includes an imaging system comprising one or more high power flash lamps fitted with special optical filters, an IR sensitive focal-plane array camera for data acquisition and a display monitor. A computer system controls the imaging system, records and analyzes surface temperature data acquired via the IR camera and provides a color or gray pattern-keyed image on the display monitor that accurately corresponds to thickness of the object.
The acquisition of surface temperature data is initiated by firing the flash-lamps to illuminate the surface of the object. The special optical filters are spectrally tuned to absorb and/or reflect all 3-5 micron IR radiation back into the flash-lamp(s). This prevents undesirable long-wave IR xe2x80x9cafterglowxe2x80x9d emissionsxe2x80x94typically generated by overheated metallic elements in the flash-lamps after the lamps are extinguishedxe2x80x94from reaching the object or the camera. The use of such filters enables a more precise thermal evaluation that can produce dimensional measurements within an accuracy range of 1%-3% of actual thickness.
A predetermined number of image frames are then recorded over a period of time after the flash lamps are fired and the recorded images used to develop a temperature-time (T-t) history for every elemental region or xe2x80x9cresolution elementxe2x80x9d over the region of interest on the object surface. Each recorded image frame is comprised of a predetermined nxc3x97m array of image pixels whose intensity correlate to the surface temperature of the object at the time the frame data was acquiredxe2x80x94each pixel having an (x,y) location designation within the image frame that corresponds to a particular resolution element.
A heat flow analysis of the T-t history is then conducted for each pixel in the acquired image frames to determine the thickness of the object at each resolution element location. Conventionally, analysis of transient heat flow through solid portions of an object requires determining a characteristic time, Tc, required for a xe2x80x9cpulsexe2x80x9d of thermal energy to penetrate the object at a first surface, reflect off an opposite surface and return to the first surface. Since this characteristic time is related to the distance between the two surfaces, it can be used to determine the thickness of the object between the two surfaces at a desired point. Because Tc is also related in time to the occurrence of an inflection point, tinfl, in the contrast-versus-time data history of a pixel according to Equ. (2) above, a value for characteristic time Tc may be determined by using a recorded intensity-versus-time history of the pixel to compute contrast-versus-time data for the pixelxe2x80x94which in the present invention is accomplished by subtracting the xe2x80x9csyntheticxe2x80x9d thermal reference T-t data from the recorded intensity-versus-time data of the pixel.
Using the synthetic thermal reference, a contrast-versus-time curve is determined for each (x,y) pixel location corresponding to each resolution element of the object surface. Next, Gaussian temporal smoothing of the pixel contrast curve data is employed to improve the signal-to-noise ratio of the measurements. The mathematical derivative of the contrast curve is then computed to identify an inflection point in the data. This derivative is preferably computed using a three-point data sampling having a first and third sample point separation that is proportionally related to the value of the image frame number at the second sample point. Next, all local xe2x80x9cpeaksxe2x80x9d in the contrast curve obtained from the derivative computation are identified and a weighting function is used as a filter to adjust the significance of localized each of these peaks to identify the actual inflection point in the T-t contrast curve data for use in determining object thickness. Finally, thickness of the object at a location corresponding to each pixel is quantitatively determined according to Equ. (1) and Equ. (2) above.